Cindy and Fred Warmbier at the UN headquarters in New York last year. They said: ‘We have been respectful during this summit process. Now we must speak out.’
Photograph: Shannon Stapleton/Reuters

The parents of Otto Warmbier, a young American who was detained by North Korea for more than a year and died soon after his release in 2017, have rebuked Donald Trump’s defense of Kim Jong-un for the death of their son.

Trump: I took Kim at his word over Otto Warmbier's torture

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“We have been respectful during this summit process. Now we must speak out,” Fred and Cindy Warmbier wrote in a statement Friday, the morning after Trump returned from his summit with the North Korean leader in Vietnam, where the pair failed to reach a deal over US sanctions and nuclear weapons.

“Kim and his evil regime are responsible for unimaginable cruelty and inhumanity. No excuses or lavish praise can change that.”

Trump received a fierce backlash, including from fellow Republicans, after he refused to blame Kim for Warmbier’s death, in response to a question during the president’s visit to Hanoi earlier this week, about how it affected his relationship with Kim.

“He [Kim] tells me that he didn’t know about it, and I will take him at his word,” Trump said at a press conference in Hanoi on Thursday. “I don’t think the top leadership knew about it.”

Warmbier, a college student, was sentenced to 15 years of hard labor after North Korea found him guilty of “crimes against the state” for allegedly taking a propaganda poster on a visit with a tour group.

He was released in June 2017, after more than a year in prison, but came home to his family in Ohio in a vegetative state. He died six days later in Cincinnati. A judge later ruled that North Korea was liable, in a largely symbolic court case that included court filings about Warmbier’s horrific physical state when returned to the US. The filings indicated he had been tortured, possibly by electric shock, and suffered severe brain damage. The cause of the brain damage was not able to be confirmed, experts said.

Otto Warmbier at a news conference in Pyongyang, North Korea in a photo released on 29 February 2016. Photograph: Kyodo/Reuters

John Sifton, the Asian advocacy director for Human Rights Watch, told the Guardian it was dangerous for Trump to believe what North Korean leaders say since Pyongyang does not allow international observers, like the United Nations, to observe their prisons and nuclear sites.

“Nothing the [North Korean] government says can be trusted,” Sifton said.

On Friday, Trump adviser Kellyanne Conway told Fox News that Trump blames the North Korean regime for Warmbier’s death – but not Kim himself.

Conway reiterated that Trump said Kim had told him in Hanoi that he did not know how Warmbier was treated in detention.

Conway said she had just spoken to Trump about it. “The president agrees with the Warmbier family and holds North Korea responsible for Otto Warmbier’s death … [he] was talking about, that Chairman Kim did not know what happened to Otto at the time when it happened.”

Nikki Haley (@NikkiHaley)

Americans know the cruelty that was placed on Otto Warmbier by the North Korean regime. Our hearts are with the Warmbier family for their strength and courage. We will never forget Otto. ❤️🇺🇸